1. Field of the Invention
The present invention relates in general to local and metropolitan area networks and, in particular, to a method for selectively eliminating latencies in the exchange of control messages between bridges during the re/convergence of the network.
2. Description of Related Art
Computers are often connected together through a network (e.g., Local Area Network (LAN), Metropolitan Area Network (MAN)) that is made up of bridges (or any other device operating at layer 2 of the Open Systems Interconnection reference model) compliant with the IEEE 802.1 Rapid Spanning Tree Protocol (RSTP). The network provides for redundant connections where data transmitted from one bridge to another bridge is constrained to follow a loop-free path. To accomplish this the RSTP uses bridges within the network to reduce the physical topology of the network to an active topology that is both loop-free (‘tree’) and fully-connected (‘spanning’). Unfortunately, the bridges can suffer from undesirable latencies when they have to communicate with one another in order for the topology to re/converge towards a spanning tree as directed by the RSTP.
The RSTP currently avoids these latencies instead of eliminating these latencies as done by the present invention. The avoidance is built into the current RSTP whose operation depends on the exchange of control messages known as Bridge Protocol Data Units (BPDUs) between bridges. Bridges are interconnected to one another by links. And, the BPDUs are transmitted on a link by a port of one of the bridges. The current RSTP limits the number of BPDUs that can be transmitted by a port. In particular, when the number of transmitted BPDUs per port (txCount) reaches a limit of txHoldCount BPDUs, the current RSTP prevents the next BPDU from being transmitted until txCount is decremented. Such a situation can create an undesirable latency.
The current RSTP is not good enough because its solution is not always successful at totally avoiding such latencies. As a consequence, the number of transmitted BPDUs is limited. In particular, the current RSTP increments txCount by 1 for each transmitted BPDU and decrements txCount by 1 at one-second intervals. If txCount is null, as many as txHoldCount BPDUs can be transmitted before txCount is decremented. If txHoldCount BPDUs have been transmitted and txCount has not been decremented, a port consequently has to wait at worse 1 second before the next BPDU can be transmitted. This is an undesirable feature of a transmission rate limiter like txHoldCount because BPDUs may not be immediately transmitted, which does not contribute to the re/convergence of an unstable topology towards a stable topology (a spanning tree). A delay of up to one second on a single port may translate into even longer delays in the re/convergence of the overall RSTP-based topology. Accordingly, there is a need for a method that can selectively eliminate such latencies in the exchange of BPDUs between bridges during the re/convergence of the network. This need and other needs are satisfied by the method, network and bridge of the present invention.